There are many known methods for treating polymer surfaces to alter their properties, including those that involve flaming, corona discharge, chemical oxidation, electrode discharge processes, or plasma treatment in the presence of specific gases and chemical species, or various combinations of these processes. All of these known processes have certain limitations, including difficulty in maintaining quality control at desirable production speeds, deleterious effects on the polymer substrate, or ineffective or inadequate results with particular polymeric substrates. Chemical oxidations, for instance, are generally wet processes and, therefore, are relatively slow and have all the attendant problems connected with coating, cleaning and drying materials so treated. Flame treatments are also slow and often adversely affect the bulk properties of the material being treated, particularly if not very carefully controlled, and of course, they also present a constant fire hazard. Corona discharge treatments involving potential gradients across the material being treated often cause pinholes in the material and induced electrostatic charges which raise many problems for post-treatment handling. Electrode discharge systems are hard to maintain because, in the presence of organic materials, one of the typical effects of such discharges is the formation of polymeric films on the electrodes. Such discharge systems, therefore, require constant cleaning of electrodes to remove the polymeric film which acts as an insulator, thereby slowing the flow of the current. A simple and effective gas plasma treatment to improve paint adhesion is not readily available, especially for articles based on or coated with polyolefins, polycarbonate or polyvinyl chloride (PVC).
U.S. Pat. No. 4,072,769 (D. D. Lidel) refers to a previously known method in which polymeric surfaces are bathed in an atmosphere of nitrous oxide (N.sub.2 O) at elevated temperatures and in the presence of ultraviolet radiation. The result of this process is said to be similar to the flame, chemical and corona discharge processes referred to above, namely, the ultraviolet radiation breaks up other bonds as well as carbon-hydrogen bonds (even carbon-carbon bonds) causing relatively severe degradation of the surface of the polymer.
U.S. Pat. Nos. 4,072,769 and 3,761,299 disclose a process for modifying the surface characteristics of polymeric materials by exposure of the materials to a reactive gas which has been activated by radio frequency electromagnetic radiation prior to the gas being directed to the polymer surface. The references teach that the invention is based on the discovery that when certain specific activator gases, i.e., the noble gases and nitrogen, when activated by radio frequency electromagnetic radiation, produce free radical sites when brought into contact with polymer surfaces and also produce free radicals from the organic or inorganic vapors (e.g., water) introduced into the activator gas stream (the organic or inorganic vapors are referred to as reactive gases). When a gas stream combining both the activator gas and reactive gas are discharged onto a polymer surface, a reaction is reported to occur between the free radicals generated at the polymer surface by the activator gas and in the reactive gas by the activator gas to provide the desired result. Certain reactive gases are said to provide satisfactory results even in the absence of an activator gas. Excellent results are said to be obtained by using certain inorganic gases alone, namely, nitrogen trioxide (N.sub.2 O.sub.3) and the "odd molecules" nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2). The reference suggests that similar results should be obtained using a reactive gas consisting of any other odd molecules (e.g., ClO.sub.2, O.sub.2, OF, etc.) which exist naturally with 3-electron bonds. In addition, water vapor alone produces satisfactory results. Alternatively, vapors of "many" organic compounds are said to be useful as the reactive gas, but only in combination with at least one of the activator gases prior to activation by the electromagnetic field.
The Lidel reference also refers to a prior art treatment for increasing the hydrophilicity of materials as disclosed by J. S. Hayward in U.S. Pat. No. 3,526,583. Lidel states that, according to the Hayward process, normally hydrophobic polymer surfaces can be rendered hydrophilic when bathed, in the presence of air, in a stream of an activated species of one of the noble gases, or of hydrogen, nitrogen, or oxygen (the latter gas being by far the least effective). It is said that the Hayward process indicates that activated gas species will attack polymer surfaces in a relatively gentle manner to cause some changes in the surface molecules.
Surface modification of polyethylene is utilized by another prior art process known as Casing-Crosslinking by Activated Species of Inert Gases (Chem. and Eng. News. Vol. 44, Sep. 26, 1966, pgs. 58 and 59, by Hanson et al.) which applies electronically excited species of inert gases (helium, argon, krypton, neon and xenon) to the surface to increase the cohesive strength of the surface molecules. The reference discussing the Casing process suggests that such activated gases do not necessarily change the wettability of the polymer surface for water, but that the activated gas attacks the surface of a polymer in a relatively gentle manner to form free radical sites. However, the reference states that Casing does not strengthen adhesive joints made with polypropylene and the evidence shows that both crosslinked and degraded polymer is formed at the surface, resulting in little change in cohesive strength of the surface region.
U.S. Pat. No. 4,276,138 to M. Asai et al. (the '138 patent) discloses a method for reducing static electricity on the surface of a shaped article made of PVC resins which comprises blending a surface active agent with the PVC resin prior to fabrication of the article and subjecting the article to treatment with a low temperature plasma gas. The incorporation of a surface active agent is critical to the invention. The gases include helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon monoxide, carbon dioxide, hydrogen, halogens, halogen compounds, olefins, halogenated hydrocarbons, aromatic hydrocarbons and heterocyclic organic compounds organosilanes. Inorganic gases are preferred, especially helium, argon, carbon monoxide, carbon dioxide, and hydrogen because of higher efficiency due to an unknown mechanism. When mixtures of gases are used (optionally), it is recommended that one of the components is carbon monoxide (CO).
U.S. Pat. No. 4,247,440 to M. Asai et al. discloses a method for preventing plasticizer bleeding on PVC shaped articles. The method requires the use of at least 20 parts of plasticizer per 100 parts of PVC which plasticizer must include at least 10% of a compound having at least one aromatic nucleus in a molecule (mixtures of plasticizers can be used). The low temperature plasma gases which are useful in the process include the same list of gases as in the '138 patent with the same preferences for inorganic gases and carbon monoxide. U.S. Pat. No. 4,272,464 to M. Asai et al. also deals with a method for preventing plasticizer bleeding, but it requires the blending of a urethane elastomer with the PVC resin prior to fabrication and plasma treatment. Again the same gases and preferred gases are disclosed as in the '138 patent.
Improved surface properties are said to be obtained with PVC articles according to U.S. Pat. No. 4,247,577 to K. Imada et al. when a covering layer of a curable organopolysiloxane composition is placed on the surface of the article after it is treated with a low temperature plasma gas. Suitable gases are helium, neon, argon, nitrogen, oxygen, air, nitrous oxide, nitrogen dioxide, carbon monoxide, carbon dioxide and hydrogen sulfide.
U.S. Pat. No. 4,302,307, also to K. Imada et al., discloses a treatment of PVC gramophone records in a low temperature plasma gas to improve antistatic properties. The gases are selected from inorganic or inert gases such as helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, chlorine, hydrogen chloride,, carbon dioxide and hydrogen. It is reported that the gases can be used singly or in mixtures, but that argon or argon-containing mixed gas is preferred because of higher efficiency.
U.S. Pat. No. 4,315,808 to K. Imada et al. discloses a method for modifying the surface properties of shaped PVC articles with a low temperature plasma to prevent bleeding of plasticizer or other additive ingredients in the shaped article. The method requires intermittent exposure to the gas plasma (at least five exposure and repose times of specified duration) rather than continuous exposure. The list of useful gases is extensive, but limited to inorganic gases including helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon monoxide, carbon dioxide, hydrogen, chlorine, halogen compounds, such as hydrogen chloride, bromine cyanide and sulfur compounds, such as sulfur dioxide, and hydrogen sulfide. It is also suggested that gases may be used singly or in mixtures, but that the oxygen-containing compounds are less preferred, e.g., oxygen, air and nitrogen oxides, as well as halogen compounds and sulfur compounds.
U.S. Pat. No. 4,396,641 to K. Imada et al. discloses a method for improving the surface properties of shaped articles of synthetic resins which is said to be applicable to not only PVC, but to other kinds of synthetic resins including thermoplastic and thermosetting resins such as low- and high-density polyethylenes, polypropylenes, polystyrenes, etc. However the gaseous composition of the plasma atmosphere requires the presence of a specific organic silicon compound. Improved results are said to be obtained when the organic silicon compounded is diluted with an inert inorganic gas selected from nitrogen, nitrogen oxides and helium, argon, neon and xenon. Furthermore it is necessary that the plasma treated article be contacted with a halogen or a halogen containing inorganic or organic compound.
U.S. Pat. No. 4,337,768 teaches that it is well known that crosslinked thin layers on the surfaces of chlorine-containing vinyl polymers, such as PVC and polyvinylidene chloride, formed by glow discharge or UV radiation, act as barriers to migration of lower molecular weight substances such as monomers, plasticizers and additives to the surface. However the method disclosed in this reference includes a gas mixture for use in the "glow discharge" consisting of carbon monoxide (CO) and at least one different gas selected from the group consisting of argon, nitrogen, carbon dioxide, water, etc. A gas mixture of CO and CF.sub.4 is characteristic. The use of a mixture of CO and H.sub.2 O is especially preferred for producing film useful in blood bags. The modified surface layer is described as having a specified thickness, being crosslinked and having reduced chlorine content (45% or less) compared to the uncrosslinked portion.
Treatment of a fluorocarbon polymer with a gas plasma to improve surface adhesive properties (i.e., heat sealing) is disclosed in U.S. Pat. No. 4,735,996. However, two critical process elements are noted: (1) the power density of the plasma is preferably 0.03 to 10 W.sec/cm.sup.2 in order to control the concentration of fluorine atoms on the surface of the polymer to within particular limits, and (2) the gas composition which is selected from Ar, H.sub.2, CO, CO.sub.2, NH.sub.3, SO.sub.2, HCl, freon gases such as CF.sub.4, H.sub.2 S and mixtures with other unspecified gases. However, the oxygen content of the gas has to be less than 10 mol %, or have a CO content of more than 10 mole %, or with an NH.sub.3 content of more than 0.1 mole %.
U.S. Pat. No. 4,828,871 discloses that shaped polymer articles, for example, polypropylene films, can be made more receptive to organic coatings (such as pressure sensitive adhesives) by exposure of the article to an electrical discharge in the presence of a chlorocarbon or chlorofluorocarbon gas, thereby generating a chlorine-containing surface layer.
U.S. Pat. No. 5,152,879 discloses a process for the low pressure plasma treatment of polyolefin films using oxygen-containing gases, such as O.sub.2, H.sub.2 O.sub.2 , H.sub.2 O, N.sub.2 O, NO.sub.2 or O.sub.3 and mixtures with noble gases such as He, Ne, Ar, Kr or Xe. The process is said to be particularly useful for preparing multilayer films. Important process parameters for improved bonding or adhesion of the film are reported to be (1) maintaining the energy density on a unit surface of polyolefin film at or above 0.01 or above 10 Ws/cm.sup.2, (2) keeping the polyolefin film at a distance of at least 60 mm from the electrodes to which the electric field for the production of the plasma is applied and (3) maintaining the temperature of the film at or below 30.degree. C. during the treatment, preferably from -2.degree. C. to 10.degree. C.
Plasma treatment of highly oriented polyolefins having an ultrahigh molecular weight to produce articles having good wetting and adhesion properties without reducing their tensile strength is carried out with inert and/or reactive gases or gas mixtures, the use of reactive gases being preferred. Suitable inert gases are nitrogen and helium, and suitable reactive gases include air, oxygen, carbon dioxide and ammonia. Preferably chemical treatment of the polyolefin is carried out immediately after plasma treatment to improve the wetting and adhesive properties of the plasma treated articles. Chemical treatment is proposed using various broad classes of reagents, including those with carboxyl groups, hydroxyl groups or carbonyl groups.
In an effort to improve the paintability of polyvinyl halide compositions, U.S. Pat. No. 5,198,303 teaches that a copolymer of vinyl halide (vinyl chloride) and an adhesion-promoting comonomer is used to produce a flexible or semi-rigid composition. The reference acknowledges that plasticizer migration in PVC polymers is an inherent limitation when good paint adhesion is required. Avoiding highly flexible PVC compositions permits the inventors to eliminate plasticizer from the composition, and copolymerizing with an adhesion promoting comonomer is said to further improve adhesion.
U.S. Pat. No. 5,169,675 describes a process for the bonding of high nitrile resins to the surface of plasma-treated plastics. The "plastics" (polybutadiene rubber is also disclosed) include polyethylene, polypropylene and PVC. The reactive gas plasma is disclosed as a single gas or combination of gases including water, hydrogen, oxygen, volatile non-polymerizing alcohols and non-polymerizing organic acids, most preferably oxygen, water or combinations thereof; but not including nitrogen or acetonitrile.
The use of cold gas plasma to treat polyolefins surfaces for improved paintability is described in articles by S. L. Kaplan, et al. ("Commercial Plasma Processes For Enhanced Paintability Of TPO Auto Fascia", "Successful TPO Painting-Cold Gas Plasma Advances Painting Application", and "Plasma Surface Treatment Of Plastics To Enhance Adhesion: An Overview"). These articles comprehensively describe the conditions and practical advantages of the plasma treatment process, and consequently reinforce the conclusion that the unique advance described in the present invention was missed by prior investigators.
However effective the referenced treatments are initially, with certain materials, such as (PVC), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene (PP) and other olefin polymers, the treatment's effectiveness deteriorates rapidly over time. This is especially true of polymers containing plasticizers, processing aids, internal lubricants, heat or UV stabilizers, or dispersion aids which can and tend to bleed (bloom, migrate) to the surface. This invention proves particularly effective for such classes of materials, which includes, but is not limited to, PVC, LDPE, LLDPE, PP and other polyolefin materials.